The black film is likely a sooty mold that grows on the honeydew produced by aphids, mealybugs or scale insects. As these pests feed they excrete a sticky substance calley honeydew which turns black as it ages. To control the problem, you need to control the insects. Once you identify the pests you can choose an insect-specific pesticide. Once you get rid of the pests you can wash off the sticky coating or simply allow it to weather away.